Tabulate corals were always colonial, and the individual polyps tended to be small. At various times in the past it has been suggested that they were not real corals, but recent work on their detailed skeletal structure shows that this is their true affinity. Preserved polyps from the tabulate genus Favosites have been discovered. Each polyp had 12 tentacles, and a similar overall appearance to the polyps of modern corals.
Tabulate corals first appear in Lower Ordovician rocks from North America, which was a low latitude continent at that time. They diversified rapidly in the Ordovician and quickly spread worldwide. A rapid radiation was followed by extreme decline in the end-Ordovician mass extinction. They recovered from this to reach a diversity peak in the Middle Devonian, but their recovery from the late Devonian extinctions was restricted and they survived with limited diversity until the end-Permian extinction.
Large tabulate corals tend to be associated with Lower Palaeozoic reefs and small ones with deeper water facies. In particular, tabulates characterize reefs built by stromatoporoids (Chapter 3), which seem to have created many niches for them to occupy. This group of sponges became extinct in the late Devonian and its absence may explain the failure of tabulate corals to return to their previous diversity after this time.
Tabulate skeletons are made of calcite and tend to be very solid in form. The most diagnostic elements of the tabulate corals are the structures developed within the corallite
(Fig. 4.4). These are dominated by the horizontal tabulae and dissepiments. Septa are short or absent. In some more advanced tabulates, the outer walls of the corallite can be thinned, or replaced completely by a marginal zone, shared between polyps, and filled with a boxy framework of internal struts, known as coenenchyme. Where corallites do have walls, they are usually perforated by mural pores, which would have allowed direct connections between the soft tissues of adjacent polyps. These adaptations indicate that there was a high degree of interconnection between the individuals making up a tabulate colony.
The overall shape of the colony was controlled partly by the substrate and partly by the pattern of addition of new corallites. These were either added around the edge of the colony, a pattern known as peripheral growth, or in between corallites, a pattern known as medial growth. Some species typically grew in only one pattern, while others could vary their pattern depending on the environmental conditions they experienced. Peripheral growth formed flat (tabulate) or low, dome-shaped colonies. Medial growth formed higher domes or nodular colonies. In a well-developed reef system, colonies in deeper water generally showed peripheral growth. Corals on the reef margin were dominated by medial growth and corals in the core of the reef showed both strategies, leading to a diverse array of colony shapes.
Corallites: individual skeletal elements occupied by one polyp. These tend to be small in tabulate corals, and to lack complicated internal structures
Individual corallites are linked into a corallum shaped like a chain (cateniform). The shape of the corallite and the corallum are highly variable in corals
Septa: small or absent in tabulate corals. This example shows small septa that grew a short distance from the corallite wall
Tabulae: horizontal plates which cut the corallite into a series of chambers. These represent the base of the section of the calice occupied by the polyp at different times during its development
Coenenchyme: shared calcareous tissue that conjoins corallites in highly interconnected colonies
Corallum is built of calcite and is a solid structure
Fig. 4.4 Main features of the hard-part morphology of tabulate corals: (a) Halysites, and (b) Heliolites. Corallites of both of these genera are between 2 and 6 mm in diameter.
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